In electrohydraulic valve control systems that embody a valve coupled to a hydraulic actuator, it is desirable to monitor position of the actuator piston for purposes of closedloop servo control. U.S. Pat. No. 4,749,936 discloses an electrohydraulic valve control system in which a coaxial transmission line is formed within the actuator to include a center conductor coaxial with the actuator and an outer conductor. A bead of ferrite or other suitable magnetically permeable material is magnetically coupled to the piston and surrounds the center conductor of the transmission line for altering impedance characteristics of the transmission line as a function of position of :he piston within the cylinder. Position sensing electronics includes an oscillator coupled to the transmission line for launching electromagnetic radiation, and a phase detector responsive to radiation reflected from the transmission line for determining position of the piston within the actuator cylinder. In a preferred embodiment, the coaxial transmission line includes a tube, with a centrally-suspended center conductor and a slidable bead of magnetically permeable material, projecting from one end of the actuator cylinder into a central bore extending through the opposing piston. In another embodiment, the outer conductor of the transmission line is formed by the actuator cylinder, and the center conductor extends into the piston bore in sliding contact therewith as the piston moves axially of the cylinder. The systems so disclosed provide improved economy and performance as compared with previous devices for a similar purpose, but are susceptible to temperature variations within the actuator, and consequent changes in properties of the dielectric material within the transmission line.
U.S. Pat. No. 4,757,745 discloses an electrohydraulic valve control system that includes a variable frequency rf generator coupled through associated directional couplers to a pair of antennas that are positioned within the actuator cylinder. The antennas are physically spaced from each other in the direction of piston motion by an odd multiple of quarter-wavelengths at a nominal generator output frequency. A phase detector receives the reflected signal outputs from the directional couplers, and provides an output through an integrator to the frequency control input of the generator to automatically compensate frequency of the rf energy radiated to the cylinder, and thereby maintain electrical quarter-wavelength spacing between the antennas, against variations in dielectric properties of the hydraulic fluid due to changes in fluid temperature, etc. A second phase detector is coupled to the generator and to one antenna for generating a piston position signal. The output of the second phase detector is responsive to phase angle of energy reflected from the piston and provides a direct real-time indication of piston position to the valve control electronics. Although the disclosed system thus addresses the problem of temperature-induced variations in electrical properties of the hydraulic fluid, a problem remains in that temperature compensation is essentially confined to fluid in the volume immediately surrounding and between the antennas, and thus does not take into consideration temperature and temperature gradients in the hydraulic fluid throughout the cylinder.
A general object of the present invention, therefore, is to provide apparatus for determining position of a piston within an electrohydraulic actuator that is inexpensive to implement, that is adapted to continuously monitor motion in real-time, that is accurate to a fine degree of resolution, and that is reliable over a substantial operating lifetime. Another object of the invention is to provide apparatus of a described character that automatically compensates for variations in dielectric properties of the hydraulic fluid due to temperature variations and gradients, etc. throughout the entire cylinder.
A further object of the invention is to provide a coaxial transmission system that embodies enhanced capability for matching impedance of a transmission line to impedance of the energy-launching antenna and associate circuitry.
Yet another object of the invention is to provide a system of general utility for monitoring position of a piston within a cylinder, and having particularly application for monitoring piston position in an electrohydraulic servo valve and actuator system of the character described.